A raw product stream (raw syngas) is generated by UCG of which syngas is a major and desirable constituent. Syngas will contain different proportions of CO, H2, CO2, N2, CH4, water and gaseous hydrocarbons depending on various factors including the type of oxidant used for UCG, water influences (both ground water and exogenous water), coal quality, and UCG operating temperature and pressure. Furthermore, a raw UCG product stream generated soon after igniting a coal gasifier or when decommissioning a coal gasifier will tend to have an unsuitable consistency (chemical or calorific) for energy or chemical production.
The raw UCG product stream will contain components that must be removed or otherwise treated prior to being suitable as a syngas stream (treated UCG product stream) to be used as a feedstock for gases to liquids (GTL), hydrogen cell or power production.
A typical raw UCG product stream will contain:    main syngas components (CO, H2, CO2, N2, CH4);    water;    minor components such as C2-C6 hydrocarbons, oxygen and argon; and    contaminants such as sulphur containing components (eg. H2S, COS, CS2, mercaptans), nitrogen based components (NH3, HCN), hydrocarbon components (coal condensate and phenols), trace components such as heavy metals (arsenic and mercury), and chlorides.
These contaminants originate from the coal itself or its gasification, or from the exogenous oxidant. Some contaminants are particularly inhibitory to downstream catalytic reactions, such as in the GTL process, in which case it is vital that they be removed from the raw UCG product stream or otherwise treated. In some instances, however, the contaminants themselves may be isolated, refined and worked up as commercial by-products (eg. natural gas, methanol, ammonia, elemental sulphur, oxo alcohols and carbon dioxide).